compositor: Handle GL video memory purged errors

Emit a signal so that interested parties can recreate their FBOs and
queue a full scene graph redraw to ensure we don't end up showing
graphical artifacts.

This relies on the GL driver supporting the
NV_robustness_video_memory_purge extension and cogl creating a
suitable GL context. For now we only make use of it with the X backend
since the only driver with which this is useful is NVIDIA.

https://bugzilla.gnome.org/show_bug.cgi?id=739178

src/backends/x11/meta-backend-x11.c

@@ -884,6 +884,8 @@ meta_backend_x11_init (MetaBackendX11 *x11)
 {
   MetaBackendX11Private *priv = meta_backend_x11_get_instance_private (x11);
 
+  clutter_x11_request_reset_on_video_memory_purge ();
+
   /* We do X11 event retrieval ourselves */
   clutter_x11_disable_event_retrieval ();